The XP Backfire is a fair bit of a joke, but the idea of shooting backwards actually appears in real weaponry.
Recoilless rifles fire something (sometimes water) backwards to counteract recoil. I saw a talk on them at the engineering conference where I presented my work on water jet trajectory and range. This is an intriguing concept.
Not sure if this sort of weaponry has any applications to water guns, but I thought it was interesting and worth sharing.